This invention relates to a ghost canceller for a television receiver.
An arrangement as shown, for example, in FIG. 1 is known as a ghost canceller. Such a canceller is disclosed in Murakami et al "TV Ghost Canceller" Toshiba Review 36 Vol. No. 7, pp 625 to 630, June 1981, which is adapted to cancel out the ghost component of a received signal by preparing a ghost replica by a transversal filter 20 capable of properly setting tap gains and subtracting the gohst replica from the received signal in a subtracting circuit 26. As a reference signal for ghost detection, use is made of a step-like waveform of the leading edge of a vertical synchronizing pulse of a television signal. A ghost detector 27 in FIG. 1 is adapted to detect the reference signal (the leading edge portion of the vertical synchronizing pulse) at the output of subtracting circuit 26, identify the position, polarity, amplitude, etc. of a ghost component signal and a tap gain correcting circuit 28 corrects tap gains stored in a tap gain memory 29. The contents of tap gain memory 29 is transferred at a proper time to a tap gain holding circuit 23 which is included in transversal filter 20. The tap gain holding circuit 23 is adapted to hold fixed tap gains during one field of a TV signal. The outputs of tap gain holding circuit 23 provide control voltages to weighting circuits 22 of the respective taps of transversal filter to vary the weight coefficients (tap gain) for an input signal applied to a delay line 21. Normally, the correction of tap gains is repeated for each reception of the reference signal. By the repetitive correction the tap gains are approximated to respective optimum values, permitting the cancellation of ghost. The reference signal inputting time, as well as the above-mentioned operation of the circuit, is controlled by a timing circuit 40.
The transversal filter 20 as disclosed in the above-mentioned document is comprised of an integrated circuit, in which a charge-coupled device (CCD) is used as delay element 21 and capacitor memories (C-memory) are used as tap gain holding circuit 23. Since an appreciably greater capacitance cannot be taken with respect to a capacitor memory element due to a restricted chip area, voltage values in the memory cannot be held during the field period of the TV signal. For this reason, the capacitor memories are refreshed by voltages obtained by D/A converting the contents of tap gain memory 29 comprised of digital elements, several hundred times per one field. In this case, an image on a TV screen is likely to be disturbed due to a transient response in transversal filter 20 which occurs when the capacitor memories are refreshed.
A type of the integrated CCD transversal filter has been developed in which digital memories and D/A converters are incorporated as a tap gain holding circuit 23 with respect to the respective taps. For further detail, reference is invited to Nagashima et al "Programmable CCD Transversal Filter TL8507P" Toshiba Review Vol. 37, No. 3, pp. 190-194 March 1982. In such a digital memory-equipped transversal filter, the tap gain holding time is theoretically infinite. When the tap gains are updated once per one field, they are completely held until the next field. If, during the vertical blanking period of the TV signal, the tap gain data is transferred from tap gain memory 29 to tap gain holding circuit 23, an adverse effect of the transient response in the transversal filter resulting from the data transfer, upon the image of the TV screen, is suppressed to a minimum level. Even in this case, it is difficult to suppress below an allowable level the disturbance of the image on the TV screen by a transient response which occurs during the data transfer. This often disturbs the vertical synchronization on the TV screen. The suppression of the transient response per se below the allowable level is most desirable, but it is not easy to achieve such suppression from the standpoint of the IC manufacturing technique. Heretofore, no satisfactory IC type CCD transversal filter has been implemented.